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Date: Sun, 6 Nov 2011 04:26:51 +0000 (UTC)
From: "jinglong.liujl (Created) (JIRA)" <jira@apache.org>
To: hdfs-dev@hadoop.apache.org
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Subject: [jira] [Created] (HDFS-2542) Transparent compression storage in
 HDFS
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Transparent compression storage in HDFS
---------------------------------------

                 Key: HDFS-2542
                 URL: https://issues.apache.org/jira/browse/HDFS-2542
             Project: Hadoop HDFS
          Issue Type: Bug
            Reporter: jinglong.liujl


As HDFS-2115, we want to provide a mechanism to improve storage usage in hdfs by compression. Different from HDFS-2115, this issue focus on compress storage. Some idea like below:

To do:
1. compress cold data.
   Cold data: After writing (or last read), data has not touched by anyone for a long time.
   Hot data: After writing, many client will read it , maybe it'll delele soon.
   
   Because hot data compression is not cost-effective,  we only compress cold data. 
   In some cases, some data in file can be access in high frequency,  but in the same file, some data may be cold data. 
To distinguish them, we compress in block level.

2. compress data which has high compress ratio.
   To specify high/low compress ratio, we should try to compress data, if compress ratio is too low, we'll never compress them.

2. forward compatibility.
    After compression, data format in datanode has changed. Old client will not access them. To solve this issue, we provide a mechanism which decompress on datanode.

3. support random access and append.
   As HDFS-2115, random access can be support by index. We separate data before compress by fixed-length (we call these fixed-length data as "chunk"), every chunk has its index.
When random access, we can seek to the nearest index, and read this chunk for precise position.   

4. async compress to avoid compression slow down running job.
   In practice, we found the cluster CPU usage is not uniform. Some clusters are idle at night, and others are idle at afternoon. We should make compress 
task running in full speed when cluster idle, and in low speed when cluster busy.

Will do:
1. client specific codec and support  compress transmission.

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